This disclosure is directed to a filtration device including a first screen and then a second screen, and, more particularly, to a device for screening wastewater.
Wastewater treatment technologies have continued to advance, increasing the use of Membrane Biological Reactors (MBR's). These MBR systems are very susceptible to fouling, which has increased the level of pre-screening requirements. Fine screens of ¼″ (6 mm) slots or aperture have not proven to provide adequate screening for many membrane systems. The trend has been towards ⅛″ (3 mm) and smaller screens. Slotted screens are no longer favorable, as they provide only one dimensional screening. Perforated plates with 2 mm and 1 mm aperture openings are gaining in popularity. Screening to such a fine level presents new challenges, as these screens typically need a larger aperture screen up stream, as the very fine screens become fouled too quickly when used alone. Additionally, screens of 2 mm or smaller aperture are subject to excessive wear unless grit removal equipment is upstream of the screen. Dual stage screening is considered the best defense to protect MBR systems. The problems associated with dual stage screening are primarily the cost and size requirements associated with two screens and separate grit removal equipment.
Several styles of screens can accomplish the task of removing solids from a wastewater stream. Drum screens and center flow band screens have gained considerable popularity for these pre-MBR applications. Because they have a continuous loop of screen elements, the water is forced through the screen, with no escape routes. The removed solids have no way of passing around the screening elements, making their inherent removal efficiencies very high. Drum screens in particular are gaining particular popularity for this application. Some use slots which provide only 1 dimensional screening, but they are relative easy to clean and low on maintenance. Round aperture screens however have proven to provide higher removal efficiencies. Some configurations of drum screens flow into the center of the drum, and out the sides, while others flow from the outside of the drum into the center. Many of these drum screens require a free flowing bottom discharge of water, requiring a pumped flow, and high head loss.